Composite electrodes for crossed-field switch device and method

ABSTRACT

The crossed-field switch device has electrodes, and particularly an outer cathode electrode which in its main body has sufficient mechanical strength to serve as the device envelope and has an interior coating of a refractory material to resist sputtering. The method of producing the electrodes includes sputtering, when the anode is positioned within the cathode in normal device assembly.

United States Patent 1 Lutz et a1.

Apr. 8, 1975 COMPOSITE ELECTRODES FOR CROSSED-FIELD SWITCH DEVICE ANDMETHOD Inventors: Michael A. Lutz, Malibu; Gunter A. G. Hoffmann, LosAngeles, both of Calif.

Hughes Aircraft Company, Culver City, Calif.

Filed: Aug. 6, 1973 Appl. No.: 386,115

Assignee:

313/355 Int. Cl. l-l0lj 1/50 Field of Search 313/157, 167, 217, 355

References Cited UNITED STATES PATENTS 5/l972 Maximov 313/355 X PrimaryExaminerR. V. Rolimec Assistant Examiner-Lawrence J. Dahl Attorney.Agent, or Firm--Allen A. Dicke, Jr.; W. H. MacAllister The crossed-fieldswitch device has electrodes, and particularly an outer cathodeelectrode which in its main body has sufficient mechanical strength toserve as the device envelope and has an interior coating of a refractorymaterial to resist sputtering. The method of producing the electrodesincludes sputtering, when the anode is positioned within the cathode innormal device assembly.

ABSTRACT 4 Claims, 3 Drawing Figures tiJEHTEE 83975 3,875,905

SHEET 1 BF 2 Fig. l.

Fig. 5.

pp y Foil Bake Argon Fill Spu'r'rer Remove Foil Bake

Buck SpuHer COMPOSITE ELECTRODES FOR CROSSED-FIELD SWITCH DEVICE ANDMETHOD BACKGROUND This invention is directed to a composite electrodestructure and method of producing the structure, particularly for use incross-field switch devices.

Crossed-field electrical discharge devices were primarily laboratorycuriosities, until recent developments have shown that they are capableof carrying high direct direct currents and interrupting against highvoltages. This capability has resulted in their design into a number ofcircuit breakers. In such circuit breakers, the cross-field devicesbecome crossed-field interrupting devices which perform the function ofinterrupting direct current to result in increasing circuit breakerimpedance. Prior patents which can use suitable crossfield switchdevices as their switching elements in circuit breaker environmentsinclude K. T. Lian U.S. Pat. No. RE 27,557; K. T. Lian and W. F. LongU.S. Pat.

No. 3,641,358; M. A. Lutz and W. F. Long U.S. Pat.

No. 3,660,723. These illustrate the manner in which a crossed-fieldswitch device can be used.

Two patents which illustrate particular structure of a crossed-fieldswitch device are G.A.G. Hofmann and R. C. Knechtli U.S. Pat. No.3,558,960 and M. A. Lutz and R. C. Knechtli U.S. Pat. No. 3,638,061.These patents discuss the maintenance of pressure in the interelectrodegap during conduction. Furthermore, G.A.G. Hofmann U.S. Pat. No.3,604,977 and M. A. Lutz and G.A.G. Hofmann U.S. Pat. No. 3,678,289discuss the management and control of the off-switching of crossed-fieldswitch devices by control of the magnetic field. Continuing improvementsare being made to enhance the voltage and current capabilities, as wellas life and reliability of the crossed-field switch devices. Theelectrode surfaces, particularly the cathode surface which is exposed tohigh intensity discharge in the crossed-field switch device, should beof a material which is resistant to sputtering to maximize tube life,and resistant to the glow-to-arc transition to attain high reliability.These requirements are met by refractory materials. However, the cathodeas a whole should have enough mechanical strength to serve as a tubeenvelope. Furthermore, it should resist eddy currents which are set upby the magnetic field switching, and it should be as economic aspossible. These requirements are best met by other than refractorymaterials.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to a compositeelectrode for a crossed-field switch device and one method of making thecomposite electrode. The composite electrode comprises a main body madeof a material which is selected for mechanical and magnetic reasons onwhich is a sputtered layer of a refractory material which is resistantto degradation in the plasma environment. The method comprises placing arefractory metal layer in the position of a first electrode andsputtering from the refractory metal layer onto the surface of a secondelectrode.

It is thus an object of this invention to provide composite electrodesfor a crossed-field device. It is a further object to provide electrodesfor a crossed-field switch device, which electrodes are formed of twodifferent materials, so that each of the materials is positioned to takebest advantages of its properties. It is yet another object to provide acrossed-field switch device which has electrodes principally formed of amaterial suitable for its structural properties and coated with arefractory material especially :suited to resist performance degradationof the switch device. It is a further object to provide a method bywhich composite electrodes are formed.

Other objects and advantages of this invention will become apparent froma study of the following portion of this specification, the claims andthe attached drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational-view,with parts broken away and parts taken in section, of a crossed-fieldswitch device having composite electrodes, in accordance with thisinvention.

FIG. 2 is an enlarged sectional view, with parts broken away, of one ofthe composite electrodes.

FIG. 3 is a sequence diagram of the preferred practice of the method ofthis invention.

DESCRIPTION The crossed-field switch 10 constructed in accordance withthis invention is shown in FIG. 1. It comprises an outwardly-facingcylindrical anode 12 around which is positioned an inwardly-facingcathode 14 to define an interelectrode space 16 therebetween. The anode12 is perforated, as by holes 18 so that the interior space 20 withinthe anode is permitted to supply gas to the interelectrode space 16 toaid in maintaining conduction, as taught by G.A.G. Hofmann and R. C.Knechtli U.S. Pat. No. 3,558,960. In order to prevent a long, straightline path between the inter-electrode space 16 and the anode interiorspace 20, baffles 22 are provided. These baffles are a first row ofaxially-spaced cylindrical rings positioned within the interior of theanode and a second set of axially-spaced cylindrical rings spacedinteriorly of the first set. These baffle rings are axially offset sothat the one set covers the spaces between the rings of the other set.Thus, straight line paths are eliminated. This prevents electrons fromdirectly passing from the interelectrode space 16 into the interiorspace 20.

Cathode 14 is part of the pressure vessel 24 which provides a structuralstrength and vacuum integrity to the crossed-field switch 10 andparticularly the interelectrode space. Lower shell 26 closes the lowerend of the vacuum space and is attached to the cylindrical wall whichforms cathode 14. Any convenient supporting means, such as leg 28 can beused to support the entire structure.

Support plate 30 is in the form of a disc which closes the upper end ofthe cylinder formed by cathode l4 and seals against it. It is raiseablefor disassembly of the device. When raised, the insulator 42 and anodestructure come up out of the cathode tank. Stand-off bushing 32 is ofinsulator material. Anode connection 34 is vacuum-sealed with respect tothe bushing and extends out of the end thereof for connection into acircuit. The anode connection 34 is in the form of a rod which extendsdown the interior of stand-off bushing 32 into the interior of theanode. Anode connection 34 terminates in support plate 36. A pluralityof legs 38 extend from support plate 36 and are attached to anode 12 tosupport the anode. The anode baffles are supported by rods which arepositioned axially between the sets of anode baffles and are secured attheir lower ends to legs 38. The upper end of anode 12 is provided witha re-entrant curved section 40 which engages against the outer surfaceof insulator tube 42 which descends from support plate 30 and whichcarries support plate 36. Spacer 44 is at cathode potential andelectrically floating electrode 46 fills the gap to prevent Paschenbreakdown.

A magnetic field is necessary to interengage with the electrical fieldin the interelectrode space to provide the crossed-field low pressureplasma discharge in the interelectrode space. The main magnetic field isprovided by field coil 48 which extends circumferentially around theinterelectrode space 16. In the particular example, a l-turn field coilis provided. By passing 20 amperes through the main field coil 48, aninterelectrode space magnetic field of 80 GAUSS is provided so thatconduction "conditions exist when the helium gas pressure in' theinterelectrode space is about 0.05 TORR and anelectric field of about250 volts per centimeter is present. In this situation, the magneticfield is said to be above the critical value so that conduction can takeplace.

In order to quickly turn off the magnetic field to stop conduction ofthe crossed-field switch, a switch coil is n'e cessaryQSwitch coil 50 isa one-turn coil. When it is energized to buck the field coil 48, itdrives the net magnetic field below the critical value so that thedevice becomes nonconducting. See M. A. Lutz and G'. A.G.'Hofrnann, U.S.pat. No. 3,678,289. Thus, the crossed-field switch device is anoff-switch. Problemsarise, if the switch tube dimensions are large andthe vacuum envelope is metallic tank, as in the present case. Theattenuation of the magnetic field diffusing through the, tank wall isconsiderable and requires a relatively high outside field to achieve thenecessary field .strength in the interelectrode space.

:that only moderate insulation requirements are present. in thepreferred embodiment, the off-switching field ,coil 50 is a single turnof aluminum with an anodized surface for insulation. A driving voltageof 4,000 volts and a pulse current of 20,000 amps is sufficient to drivethe netmagnetic field below the critical value. A fast rise time of theswitching pulse can be achieved with relatively low driving voltages.

Insulating end rings 56 and 58 support the cylindrical parts of the coilstructures. Off-switching coil 50 is supported closely adjacent theouter surface of the cylindrical tank wall which forms cathode 14. Next,the short circuit winding 52 is positioned around the switch coil 50.Next, insulating mandrel 54 is positioned around the short circuitwinding. Finally, the primary field winding 48 is wound around themandrel 54. An electrostatic shield 60 surrounds the field coils tominimize electromagnetic interference from the coils onto the adjacentexterior spaces.

FIG. 2 shows one of the electrodes of the crossedfield switchdevice vl0.While the. electrode illustrated in FIG. 2 could be either of the anodeor cathode electrodes, cathode l4'is illustrated. The electrodecomprises a body, material 62 and a coating material 64. The electrodeas-a whole must meet particular requirements. For example, the cathode14 should have enough mechanical strength to serve as the tube envelope.Furthermore, it should resist the eddycurrents which are set up by themagnetic field switching. Also, the body material should be aseconomical as possible. These three requirements are best met bynonrefractory materials. The body material thus can be broadly definedas a non-refractory solid electrical conductor. In order to be furtherdefinitive and not limitative, the following body materials form apreferred group: graphite, stainless steel, or aluminum.

It should be emphasized that the body material 62 is desirably of higherelectrical resistivity than thedeposited material, because eddy currentsset up in the relatively thick body material severely impede thediffusion of the magnetic field into the tube, field inhomogenieties cancause the plasma to concentrate in localized areas and lead to aglow-to-arc transition. Arc discharge is undesirable, because it cannotbe offswitched by decreased magnetic field. A reasonable magneticdiffusion time is less than 20 microseconds. Thus, in a substrate whichhas a thickness sufficient to serve as a vacuum envelope, for example a3 millimeter thickness, a moderately high electrical resistivityisdesired. The resistivity found in type 303 stainless steel, about 72micro-ohm-centimeter is suitable.

The coating material 64 is exposed to the high intensity discharge inthe crossed-field switch device. It should be of a material which isresistant to sputtering, in order to obtain maximum tube life.Furthermore, it should be resistant to glow-to-arc dischargetransitions, in order to have high off-switching reliability. Theserequirements are best met by refractory materials, such as hafnium,tantalum, tungsten, zirconium and rhenium. Several methods are availableto deposit the coating material 64 onto the substrate 62. A few onlyshould be cited: plasma spraying, chemical or normal vapor deposition,and explosively bonding.

One preferable process by which the deposition is accomplished issputter deposition. The first step of this process is to place a layerof refractory metal in the position of one of the electrodes and sputterit onto the.

other electrode. Thus, the above-described anode structure could beremoved by lifting it out of the top.

of the device 10, and a temporary anode of refractory material could beplaced therein as a source for material to be sputtered onto thecathode. It is much preferred, however, to simply apply a refractorymetal foil around the exterior of anode 12 so that the refractory metalfoil acts as an anode surface facing the interelectrode space. Thus, thefirst step of the preferred embodiment of the process is to apply foilaround the exterior cylindrical surface of anode 12 and to reinstall theanode structure within the cathode. The nextstep is to bake andevacuate. Baking at 200 degrees C. for 8 hours followed by evacuation tol0 Torr is satisfactory. After the evacuation, the device is filled'with argon to a pressure of 0.1 Torr.

Sputtering of the refractory metal from the foil onto the interior ofthe cathode surface is now accomplished by applying a voltage of 500 V,with the refractory foil negative. Under these conditions, there is aglow discharge in the interelectrode space. The ions attracted towardthe refractory foil cause dislodgement of refractory metal molecules(sputtering). These diffuses to the cathode and deposited thereon.Sputtering is carried on until a layer of about 40 microns of sputteredrefractory material is deposited onto the interior cathode surface.

Next, the device is dissembled and the foil is removed. Then, it isagain baked and evacuated, followed by an argon fill. The baking,evacuation and fill are under the same conditions as the steps describedabove. Now, back-sputtering is accomplished by applying aninterelectrode voltage at about the same value as before, but with thecathode negative. Sputtering of the refractory material from the coatingon the cathode to the anode is accomplished. Back-sputtering is carriedon until about half the material is sputtered onto the anode. Now, therefractory layer is about 20 microns on the cathode interior surface andthe anode exterior surface. A charge of about coulombs per square cm isnecessary to sputter each microns of thickness of refractory material.

In view of the fact that the refractory material is transferred bysputtering in glow discharge, it is clear that all of the surfaces whichare subject to the plasma in ordinary conduction are sputter-coated withthe refractory material. Thus, all surfaces which are adjacent theplasma during normal operation of the device 10 are coated withrefractory metal.

Each of the patents and other sources of information referred to aboveare incorporated herein in their entirety by this reference. Thisinvention having been described in its preferred embodiment, it is clearthat it is susceptible to numerous modifications and embodiments withinthe ability of those skilled in the art and without the exercise of theinventive faculty. Accordingly the scope of this invention is defined bythe scope of the following claims.

What is claimed is:

1. A cross-field switch device comprising:

an anode electrode and a cathode electrode withan interelectrode spacetherebetween, one of said electrodes being tubular and surrounding theother to define the interelectrode space, a gas in said interelectrodespace, said tubular electrode forming a part of a vacuum envelope tomaintain the gas at reduced pressure, means for producing a magneticfield through said tubular electrode into said interelectrode space sothat, when a voltage is applied across said interelectrode space,current is conducted between said anode and said cathode through a lowpressure plasma glow discharge, the improvement comprising:

said tubular electrode being of a structurally sound,

electrically moderately conductive body material selected from the groupconsisting of graphite, stainless steel and aluminum and a refractorycoating selected from the group consisting of hafnium, tantalum,tungsten, zirconium and rhenium thereon to form a composite electrode,said refractory coating being on the side of said coated electrodefacing said interelectrode space.

2. The crossed-field switch device of claim 1 wherein both said anodeand said cathode electrodes are composite electrodes.

3. The crossed-field switch device of claim 1 wherein said refractorymaterial layer is sputter-deposited onto said body material to form anadhered, uniform, dense coating layer.

4. The crossed-field switch device of claim 3 wherein both said anodeand said cathode electrodes are composite electrodes.

1. A cross-field switch device comprising: an anode electrode and acathode electrode with an interelectrode space therebetween, one of saidelectrodes being tubular and surrounding the other to define theinterelectrode space, a gas in said interelectrode space, said tubularelectrode forming a part of a vacuum envelope to maintain the gas atreduced pressure, means for producing a magnetic field through saidtubular electrode into said interelectrode space so that, when a voltageis applied across said interelectrode space, current is conductedbetween said anode and said cathode through a low pressure plasma glowdischarge, the improvement comprising: said tubular electrode being of astructurally sound, electrically moderately conductive body materialselected from the group consisting of graphite, stainless steel andaluminum and a refractory coating selected from the group consisting ofhafnium, tantalum, tungsten, zirconium and rhenium thereon to form acomposite electrode, said refractory coating being on the side of saidcoated electrode facing said interelectrode space.
 2. The crossed-fieldswitch device of claim 1 wherein both said anode and said cathodeelectrodes are composite electrodes.
 3. The crossed-field switch deviceof claim 1 wherein said refractory material layer is sputter-depositedonto said body material to form an adhered, uniform, dense coatinglayer.
 4. The crossed-field switch device of claim 3 wherein both saidanode and said cathode electrodes are composite electrodes.